I've figured out what the colour problem was.
The version of Opera I have on Windoze NT was set in "user" mode (meaning it uses custom colour and CSS settings instead of those of the site). Switching to "author" mode solved the problem.
The version I have on Windoze 98 was set in "author" mode (meaning it uses the colour and CSS settings defined by the site's author). That is why the colours looked okay when I checked the second time.
The JavaScript problem in IE5 remains unresolved though (some of the JavaScript works, but not all of it).
The version of Opera I have on Windoze NT was set in "user" mode (meaning it uses custom colour and CSS settings instead of those of the site). Switching to "author" mode solved the problem.
The version I have on Windoze 98 was set in "author" mode (meaning it uses the colour and CSS settings defined by the site's author). That is why the colours looked okay when I checked the second time.
The JavaScript problem in IE5 remains unresolved though (some of the JavaScript works, but not all of it).